Abstract Obesity is a present and increasing worldwide health threat. The vast majority of obese people contain higher levels of leptin in the circulation, which can be modeled in wild type mice on high fat diet (HFD) to promote diet induced obesity (DIO). Positive energy imbalance in the presence of higher leptin levels indicates reductions in leptin action, which likely contribute to DIO. In this application, we propose to gain better understanding of leptin action reduction in DIO. Major leptin target neurons that regulate energy balance are localized in mediobasal hypothalamus (MBH), which is exposed to the circulation due to the incomplete blood-brain barrier at the Median Eminence. Byproducts in circulation following HFD could therefore impair MBH neuron homeostasis, which we define as a healthy balance of post-mitotic quiescence, proliferation, survival, neurogenesis, and differentiation. The tumor suppressor pRb is a central regulator of cellular homeostasis, we propose to apply the knowledge of pRb function in tumor suppression to study homeostasis of MBH neurons in DIO. We obtained evidence that HFD induces pRb phosphorylation and inactivation in MBH neurons. We then tested the effects of expressing an un-phosphorylable pRb (pRb!P) in MBH to preserve pRb function in DIO, and found significantly reduced DIO. In this MPI RO1 application, we propose to (1) determine the mechanisms of pRb!P function in MBH to inhibit DIO, (2) determine the anti-DIO effects of pRb!P when expressed in POMC neurons, (3) identify non-POMC neurons in MBH that contribute to inhibition of DIO when pRb!P is expressed in MBH and determine the underlying mechanisms, and (4) determine the translational potential of our finding that expressing pRb!P in MBH can inhibit DIO.